Arboviruses — Questions

Hepatitis B virus is digested in the mosquito gut and never replicates in or migrates through the insect, so it is not an arbovirus even though it circulates in blood.

Yellow fever, dengue, West Nile and Rift Valley fever viruses all replicate in their vectors and are true arboviruses.

By day 10 the viraemia has usually cleared and antibody has risen, so IgM and IgG serology is the diagnostic approach.

NS1 antigen, RT-PCR and culture detect the virus directly and are reliable only in the first few days, while the patient is viraemic.

West Nile virus is transmitted by Culex mosquitoes from a bird reservoir, not by Aedes aegypti.

Dengue, Zika, chikungunya and, in its urban cycle, yellow fever are all spread by Aedes aegypti, one vector serving three virus families.

Non-neutralising antibody from a prior serotype binds a new serotype and carries it more efficiently into Fc-receptor-bearing macrophages, raising viral load and the inflammatory response.

Complement lysis, molecular mimicry and direct cytotoxicity are not the mechanism, and antibody does not durably cross-neutralise the four serotypes.

The plaque-reduction neutralisation test measures type-specific neutralising antibody and is the confirmatory gold standard when flavivirus serology cross-reacts.

Capture and antigen ELISAs and haemagglutination inhibition are prone to the same cross-reactivity that created the problem.

Humans reach a viraemia high enough to infect feeding mosquitoes with dengue, so they are the amplifying host in the urban human-to-mosquito-to-human cycle.

For West Nile, Japanese encephalitis, Eastern equine encephalitis and Sindbis viruses, humans are dead-end hosts that do not sustain transmission.

Rift Valley fever and Crimean-Congo haemorrhagic fever are Category 1 notifiable medical conditions and must be notified within 24 hours, reflecting their haemorrhagic-fever severity.

All other arboviral infections, whether endemic or imported, are Category 3 and reported through routine channels.

Crimean-Congo haemorrhagic fever virus spreads from patient to staff through blood and body fluids and has caused documented nosocomial outbreaks, so suspected cases need barrier nursing and viral haemorrhagic fever isolation.

The mosquito-borne dengue, West Nile, Sindbis and chikungunya viruses do not spread from person to person.

The Bunyavirales carry a three-segment negative-sense RNA genome and bud at the Golgi, unlike the flaviviruses (endoplasmic reticulum) and alphaviruses (plasma membrane).

Flaviviridae and Togaviridae have non-segmented positive-sense genomes; Reoviridae are segmented but double-stranded; Rhabdoviridae are non-segmented negative-sense.

Transovarial transmission passes the virus from an infected female mosquito to her eggs, so drought-resistant eggs can seed the next outbreak when the rains return.

Mechanical transmission and bridge vectoring move virus between hosts, not into eggs; antigenic shift and cap-snatching are unrelated molecular processes.

An arbovirus must replicate inside its arthropod vector, completing the extrinsic incubation period from gut to salivary glands. This biological transmission is the defining criterion of the group.

Carriage on the mouthparts is mechanical transmission, which excludes a virus. A DNA genome, faecal-oral spread and a human reservoir are irrelevant to the definition; most arboviruses have RNA genomes and non-human reservoirs.